2013 07 Transaction Isolation Level

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Isolation Levels FAQ

Transaction Isolation Levels of Database System

Aecho

Penpower, Inc

[email protected]

2013, 07

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Overview

OverviewAnomalies and Isolation Levels

AnomaliesDirty ReadsUnrepeatable ReadsPhantoms

Isolation LevelsDefault levelLocks of Isolation Levels

FAQ

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References

• Database Management Systems, 3rd edition.

• Isolation level, @wiki. http://goo.gl/NYSza

• Isolation level, @msdn. http://goo.gl/deqhV

• Acid, @wiki. https://en.wikipedia.org/wiki/ACID

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http://goo.gl/NYSza

http://goo.gl/deqhV

https://en.wikipedia.org/wiki/ACID

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Brief

ACID properties of database system.

• AtomicEach transaction is regarded as atomic.

• ConsistencyThe consistency property ensures that any transaction willbring the database from one valid state to another.

• IsolationUsers should be able to understand a transaction withoutconsidering the effect of other concurrently executingtransactions.

• DurabilityOnce the DBMS informs the user that a transaction has beensuccessfully completed, its effects should persist even ifthe system crashes before all its changes are reflected ondisk.

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Brief

Strict 2-Phase Locking1

.Rule 1..

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• Shared lock Allow another transaction to read.

• Exclusive lock No allow another transaction to read or write.

.Rule 2..

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All locks held by a transaction are released when the transaction iscompleted.

1Strict 2PL5 / 31

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Brief

About Isolation Level

• Most DBA System offers a number of transaction isolationlevels, which control the degree of locking when selectingdata.

• The higher isolation level, the more locks needed.It is trade-off.

• For concurrency control, with multiple transactions.

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Brief

Level/Locks Write Lock Read Lock Range Lock

Read Uncommitted - - -Read Committed X S -

Repeatable Read X X -Serializable X X X

• X → Exclusive Lock

• S → Shared Lock

• - → Nothing

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BriefLevel/Anomaly Dirty Read Unrepetable read Phantom

Read Uncommitted Maybe Maybe MaybeRead Committed No Maybe Maybe

Repeatable Read No No MaybeSerializable No No No

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Anomalies

• Dirty Reads

• Unrepeatable Reads

• Phantoms

A Sample User Table

id name age

1 Joe 202 Jill 25

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Anomalies

• Dirty Reads

• Unrepeatable Reads

• Phantoms

A Sample User Table

id name age

1 Joe 202 Jill 25

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Dirty Reads.

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When a transaction is allowed to read data from a row that hasbeen modified by another running transaction and not yetcommitted.

Level/Anomaly Dirty Read Unrepetable read Phantom






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Dirty Reads







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Dirty Reads

Transaction 1.

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/∗ Query 1 ∗/SELECT age FROM u s e r s WHERE i d =

1 ;/∗ w i l l r ead 20 ∗/

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/∗ Query 1 ∗/SELECT age FROM u s e r s WHERE i d =

1 ;/∗ w i l l r ead 21 ∗/

Transaction 2

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/∗ Query 2 ∗/UPDATE u s e r s SET age = 21 WHERE i d

= 1 ;/∗ No commit he r e ∗/

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ROLLBACK;

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Unrepeatable Reads

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A row is retrieved twice and the values within the row differbetween reads.







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Unrepeatable Reads







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Unrepeatable Reads

Transaction 1.

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/∗ Query 1 ∗/SELECT ∗ FROM u s e r s WHERE i d = 1 ;

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/∗ Query 1 ∗/SELECT ∗ FROM u s e r s WHERE i d = 1 ;COMMIT;

Transaction 2

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/∗ Query 2 ∗/UPDATE u s e r s SET age = 21 WHERE i d

= 1 ;COMMIT;

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Phantoms

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when two identical queries are executed, and the collection of rowsreturned by the second query is different from the first.


Read Uncommitted Maybe Maybe MaybeRead Committed No Maybe MaybeRepeatable Read No No Maybe

Serializable No No No




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Phantoms


Read Uncommitted Maybe Maybe MaybeRead Committed No Maybe MaybeRepeatable Read No No Maybe

Serializable No No No




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Phantoms

Transaction 1.

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/∗ Query 1 ∗/SELECT ∗ FROM u s e r sWHERE age BETWEEN 10 AND 30 ;

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/∗ Query 1 ∗/SELECT ∗ FROM u s e r sWHERE age BETWEEN 10 AND 30 ;

Transaction 2

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/∗ Query 2 ∗/INSERT INTO u s e r s ( id , name , age )

VALUES ( 3 , ’Bob ’ , 27 ) ;COMMIT;

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Isolation Levels

• Read UncommittedThe lowest level.

• Read Committed

• Repeatable Read

• SerializableThe highest level.

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Isolation Levels

The default isolation level.

• Sqlite• Serializable by default.2

• Able to switch to Read uncommitted.

• Mssql• Read Committed by default.3

• Serializable → WCC’s Category tree.

2Sqlite pragma statements. http://goo.gl/pYNwN

3Isolation level, @msdn. http://goo.gl/deqhV21 / 31

http://goo.gl/pYNwN

http://goo.gl/deqhV

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Isolation Levels




• X → Exclusive Lock

• S → Shared Lock

• - → Nothing

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Read Committed

• Exclusive locks

• Obtains before writing objects.• Released until the end of transaction.

• Shared locks

• Obtained before reading objects.• Released immediately.

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Read Committed

• Exclusive locks

• Obtains before writing objects.• Released until the end of transaction.

• Shared locks

• Obtained before reading objects.• Released immediately.

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Repeatable Reads and Serializable

• Obtains exclusive locks before reading or writing.

• All locks released until the end of transaction, according toStrict 2-PL.

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• Repeatable Reads → locks individual object.

• Serializable → locks set of objects.

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Repeatable Reads and Serializable

• Obtains exclusive locks before reading or writing.

• All locks released until the end of transaction, according toStrict 2-PL.

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• Repeatable Reads → locks individual object.

• Serializable → locks set of objects.

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Transaction and locks

When the transaction waits for lock resources ...

• Wait to gain the locks.

• Commit or Rollback

• Dead lock... ?

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......Set up time-out to prevent dead lock.

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• Dead lock... ?

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• Dead lock... ?

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Thinking

In practice, what isolation level do we need ?

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The End

FAQ, any questions ?

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2013 07 Transaction Isolation Level

Education

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